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<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<style>
    .txt {
        text-align: center;
        font-size: 30px;
        color: rgb(39, 43, 100)
    }
</style>
</head>

<body>
    <div class="txt">
        <h1>
            大标题
        </h1>
    </div>
</body>

</html>